Calculated vs measured scattering and kinetic data for the Li+HCl reaction

Li + HF and Li + HCl Reactions Revisited I: QCT Calculations and Simulation of Experimental Results

The Li + HF and Li + HCl reactions share some common features. They have the same kinematics, relatively small barrier heights, bent transition states, and are both exothermic when the zero point energy is considered. Nevertheless, the pioneering crossed beam experiments by Lee and co-workers in the 80s (Becker et al., J. Chem. Phys. 1980, 73, 2833) revealed that the dynamics of the two reactions differ significantly, especially at low collision energies. In this work, we present theoretical simulations of their results in the laboratory frame (LAB), based on quasiclassical trajectories and obtained using accurate potential energy surfaces. The calculated LAB angular distributions and time-of-flight spectra agree well with the raw experimental data, although our simulations do not reproduce the experimentally derived center-of-mass (CM) differential cross section and velocity distributions. The latter were derived by forward convolution fitting under the questionable assumption that the CM recoil velocity and scattering angle distribution were uncoupled, while our results show that the coupling between them is relevant. Some important insights into the reaction mechanism discussed in the article by Becker et al. had not been contrasted with those that can be extracted from the theoretical results. Among them, the correlation between the angular momenta involved in the reactions has also been examined. Given the kinematics of both systems, the reagent orbital angular momentum, l , is almost completely transformed into the rotation of the product diatom, j'. However, contrary to the coplanar mechanism proposed in the original paper, we find that the initial and final relative orbital angular momenta are not necessarily parallel. Both reactions are found to be essentially direct, although about 15% of the LiFH complexes live longer than 200 fs.

Dynamical and Mechanical Insights into the Li(2 S)+ HCl( X 1 Σ + ${X^1 {\rm{\Sigma }}^ + }$ ) Reaction: A Detailed Quantum Wavepacket Study

Quantum wave packet dynamics of the Li(² S)+HCl( X 1 Σ + ${X^1 \Sigma ^ + }$ ) reaction in its electronic ground state is studied. The initial state-selected and energy-resolved dynamical attributes such as reaction probability, integral cross section, and thermal rate constant for the Cl-abstraction and H-abstraction pathways are reported. All partial wave contributions of J up to 120 were found to be necessary for the title reaction up to the collision energy of ∼1.0 eV. The dynamical results reveal that the Cl-abstraction is more favored over the H-abstraction for the different rovibrational (v, j) excitations. Due to the existence of an early barrier in the potential energy surface, the cross sections increase with increasing collision energy. The rate constants also monotonously increase with temperature for both channels. Resonances are identified and characterized in terms of eigenfunctions and lifetimes. Nearly 120 well-resolved eigenstates are reported for the LiHCl complex, and they are categorized as van der Waals (vdW), barrier and product states according to the nodal progressions along (R, r, γ). The vdW resonances reveal a local-mode behavior of quasibound type at low energies and extended progressions at high energies. Further, the single-quantized periodic orbit type is also observed in the barrier region, which decays very fast. Finally, the lifetime analysis reveals that the vdW resonances can survive as long as ∼2.2 ps, which is much longer than the lifetime of the resonances in the barrier region.

Impact of the Long-Range Interaction on the Efficiency of the Li + ClH → LiCl + H Reaction

Quantum and quasiclassical calculations have been performed to compute the low energy efficiency of the Li + ClH → LiCl + H reaction on some potential energy surfaces fitted to ab initio electronic energies using different functional forms. The outcomes of the calculations show marked differences at threshold and in the shape of the excitation function in seeming contrast with the height of the saddle to reaction and the width of the cone of acceptance. The differences in the computed reactive probability and cross section are rationalized in terms of the attractive/repulsive nature of the long-range interaction and the inability of trajectory techniques to deal with threshold effects. The vestiges of these features in the value of the thermal rate coefficients are also commented on.

A new ab initio potential energy surface of LiClH (1A') system and quantum dynamics calculation for Li + HCl (v = 0, j = 0-2) → LiCl + H reaction

A new ab initio potential energy surface (PES) for the ground state of Li + HCl reactive system has been constructed by three-dimensional cubic spline interpolation of 36 654 ab initio points computed at the MRCI+Q/aug-cc-pV5Z level of theory. The title reaction is found to be exothermic by 5.63 kcal/mol (9 kcal/mol with zero point energy corrections), which is very close to the experimental data. The barrier height, which is 2.99 kcal/mol (0.93 kcal/mol for the vibrationally adiabatic barrier height), and the depth of van der Waals minimum located near the entrance channel are also in excellent agreement with the experimental findings. This study also identified two more van der Waals minima. The integral cross sections, rate constants, and their dependence on initial rotational states are calculated using an exact quantum wave packet method on the new PES. They are also in excellent agreement with the experimental measurements.

A new potential energy surface of LiHCl system and dynamic studies for the Li(2S) + HCl(X1Σ+) → LiCl(X1Σ+) + H(2S) reaction

A new global potential energy surface (PES) is constructed for the ground state of LiHCl system based on high-quality ab initio energy points calculated using multi-reference configuration interaction calculations with the Davidson correction. The AVQZ and WCVQZ basis sets are employed for H and Li atoms, respectively. To compensate the relativistic effects of heavy element, the AWCVQZ-DK basis set is employed for Cl atom. The neural network method is used for fitting the PES, and the root mean square error is small (1.36 × 10^-2 eV). The spectroscopic constants of the diatoms obtained from the new PES agree well with experimental data. The geometric characteristics of the transition state and the complex are examined and compared with the previous theoretical values. To study the reaction dynamics of the Li(²S) + HCl(X¹Σ⁺) → LiCl(X¹Σ⁺) + H(²S) reaction, quantum reactive scattering dynamics calculations using collection reactant-coordinate-based wave packet method are conducted based on the new PES. The results of the reaction probabilities indicate that a small barrier exists along the reaction path as observed from the PES. The integral cross section curves reveal that the product molecule LiCl is easily excited. In addition, the reaction is dominated by forward scattering, and similar pattern is observed from Becker's experiment.